We are studying 26S proteasomes in the fission yeast, S. pombe. Using a strain in which a protein component of the proteasome has been replaced by an allele tagged with HA, we have characterized the localization of this structure. Immunofluorescence microscopy, using either an HA antibody or antibodies against polypeptide subunits of the 26S complex show identical localization to discrete, punctate structures surrounding the nucleus. Immunoelectron microscopy has been used to investigate the localization of the 26S complex at higher resolution. Rapidly frozen and freeze-substituted preparations of both the wild-type and HA-containing strains have been embedded both Lowicryl and LR White. Antibodies to HA localize this epitope to the margin of the nucleus, just inside the nuclear envelope. Control cells (those without HA) lack this staining. These results suggest that the majority of the proteasomes are localized inside the nuclear envelope but closely associated with it. (Submitted to the Journal of Cell Biology) C9